measuring the human body’s micro climate using a thermal

IndoorAir24(6),567‐579(2014)

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Measuringthehumanbody’smicro‐climateusingathermalmanikin

ConradVoelker1,SilvioMaempel2,OliverKornadt31UniversityofKaiserslautern,DepartmentofModelinginBuildingPhysics,Paul‐Ehrlich‐Str.29,67663Kaiserslautern,Germany

2Ed.ZüblinAG,Mies‐van‐der‐Rohe‐Straße6,80807München3UniversityofKaiserslautern,DepartmentofBuildingPhysics/Low‐EnergyBuildings,Paul‐Ehrlich‐Str.29,67663Kaiserslautern,Germany

(Received23September2013;accepted18March2014;publishedonline25March2014)

CopyrightNotice

Copyright 2014 John Wiley & Sons. This article may be downloaded for personal use only. Any other use requires priorpermissionoftheauthorandJohnWiley&Sons.ThefollowingarticleappearedinIndoorAir24(6),567‐579(2014)andmaybefoundathttps://doi.org/10.1111/ina.12112.

AbstractThehumanbodyissurroundedbyamicro‐climatewhichresultsfromitsconvectivereleaseofheat.Inthisstudy,theair

temperatureandflowvelocityofthismicro‐climateweremeasuredinaclimatechamberatvariousroomtemperatures,usingathermalmanikinsimulatingtheheatreleaseofthehumanbeing.Differenttechniques(ParticleStreakTracking,thermography,anemometry,andthermistors)wereusedformeasurementandvisualization.Themanikinsurfacetemperaturewasadjustedtothe particular indoor climate based on simulationswith a thermoregulationmodel (UCBerkeleyThermalComfortModel).Wefoundthatgenerally, themicro‐climate is thinnerat the lowerpartof the torso,butexpandsgoingup.At thehead, there isarelativelythickthermallayer,whichresultsinanascendingplumeabovethehead.However,themicro‐climateshapestronglydependsnotonlyon thebody segment, but alsoonboundary conditions: thehigher the temperaturedifferencebetween thesurfacetemperatureofthemanikinandtheairtemperature,thefastertheairflowinthemicro‐climate.Finally,convectiveheattransfer coefficients strongly increasewith falling room temperature,while radiative heat transfer coefficients decrease. Thetypeofbodysegmentstronglyinfluencestheconvectiveheattransfercoefficient,whileonlyminimallyinfluencingtheradiativeheattransfercoefficient.

IndoorAir24(6),567‐579(2014)

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1. Introduction

Theclimatesurroundingthehumanbodyhasagreatimpactonhumanwell‐being.However,indoorclimateisalsoinfluenced by the heat‐release of the human body itself.Convective heat release from the human body creates amicro‐climateconsistingofaboundarylayersurroundingthebodyandanascendingplumeofheatabove theheadwhichbothdiffergreatly in temperatureand flow from thatof theindoorclimate.Acomparisonofallbodysegmentsshowsthatthis micro‐climate can vary greatly in thickness and shape.Murakami et al. (1997) showed that the thickness of thethermalboundarylayersurroundingthefeetiscomparativelysmall(5cm)butthatitbecomesthickerasonemovesupthebody. Large surfaces along the flow direction, such as thechestandbackarecoveredbyarelativelythicklayer,whichis associated with a low convective heat loss. The thermallayeraroundtheneckwasmeasuredat19cm.Thethicknessof the velocity boundary layer differs from the thermalboundarylayer,whichhasathicknessofaround8cmatthefeet and about 15 cm at the head. The velocity of theascending air flow reaches its maximum within the plumeabove the head, measuring 0.2‐0.3 m/s (Murakami, 2002,Murakamietal.,1999).

Undernormalconditions,theconvectiveheattransferof the human body is dominated by buoyancy‐drivenconvection. One of the driving forces is the temperaturedifference between the surface of the human body and thesurrounding air. The other primary factor is flow velocity,whichissignificantlyresponsibleforconvectiveheatrelease.The higher the flow velocity, the higher the heat transfercoefficient.Otherparameterssuchasclothingoractivityaresecondary, as they have an influence on the surfacetemperature of the human body and hence on thetemperaturedifference.

Thepropertiesofthehumanbody’smicro‐climatearehighly dependent on its boundary conditions and theparticular body segment in question. The measurementsintroduced in this paper confirm this claim.Values found inearlier research provided a good estimation, but did notsufficiently describe the micro‐climate around the humanbody.Withregardtoheating,ventilationandairconditioning,themicro‐climateof thehumanbodyhasbeen simulated tosome extent by Gao and Niu (2004), Dygert et al. (2009),Deevy et al. (2008) and Sevilgen and Kilic (2011) usingComputational Fluid Dynamics (CFD). Craven and Settles(2006) measured the human micro‐climate using humanvolunteersandaSchlierencamera. Schlieren techniquehasbeen also used by Tang et al. (2011) to investigate humanexhaled airflows for the reason of aerosol infection control.Worth mentioning is also the benchmark experiment ofNilssonetal.(2007),whousedathermalmanikininaclimatechamber.

This paper featuresnumerousnewapproaches. Firstofall,themeasurementswereconductedwithacombinationof local measurement methods (anemometry, thermistors)

andglobalmeasurementmethodssuchasthermographyandParticle Streak Tracking. Altogether, four independentmeasurementmethodswereusedtomeasureairvelocityandair temperature. Additionally, the control of the surfacetemperatureofthemalemanikinusedinthisstudy(mostoftheexistingstudiesdealwithafemalebody)wasdrivenbyacomplex human thermoregulation model. Finally, the studyalso investigates the impact of different room airtemperaturesonthemicroclimate.

The findings of this paper generate a betterunderstanding of human body’s micro‐climate. Thisknowledge is useful in fields dealing with thermal comfort,HVAC (heating, ventilation and air conditioning), indoor airquality and the spread of airborne diseases. Themeasurements can also be used for the validation of CFDsimulations.Furthermore,acomprehensiveknowledgeofthemicro‐climate surrounding the human body is necessary totransfer data from CFD simulations to a thermal comfortmodel (Voelker and Kornadt, 2010, Voelker and Kornadt,2012).

2. Measurementsetup

2.1. Climatechamber

A series of measurements were carried out in aclimate chamber (3 m x 3 m x 2.44 m). The chamber istempered using water‐bearing capillary tubes placed underthe plaster of every surface (4 walls, floor, and ceiling).Additionally, the chamber is equipped with a ventilationsystem with an adjustable air change rate and the choicebetween recirculatedor fresh air. The ventilation aswell asevery single surface can be separately controlled in atemperature range of between 10° and 40°C. Hence, theclimate chamber is suited for the investigation of typicalindoorclimates,butalsoforthestudyofphenomenasuchasthe micro‐climate surrounding the human body. Allmeasurements in the climate chamberwere performed in asteadystate,whichwasensuredbyanappropriatelead‐timeandconstantmonitoringoftheindoorclimateparameters.Inthe study, the climate chamberwas equally temperedby allsix surfaces while the ventilation system was switched off.The result was a relatively uniform climate with almost noverticalgradient.Theairtemperaturewasslightlyabovethatofthewalls,duetotheheatreleasedfromathermalmanikin.

The chamber was also equipped with a thermalmanikin (nicknamed “Feelix”). With heating wires directlyunder his surface, Feelix is able to simulate the skintemperature and sensible heat release of the human body.Themanikinwas used because the human body affects thetemperatureandflowconditionsasasourceofheataswellasanobstacletothefreeflowofair(Melikov,2004).Theuseofamanikinallowstheinvestigationoftheimpactoftheindoorclimate on the human being and vice‐versa. The use ofartificial pump‐controlledbreathingwithin themanikinwas

IndoorAir24(6),567‐579(2014) Voelkeretal.:Measuringthehumanbody’smicro‐climate

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not used during the measurements to avoid the transientcharacteristicsofinhalationandexhalation.

Themanikinwaspositionedinasittingposture,withthe outstretched arms representing aworking position at adesk. To guarantee stability, the manikin was seated on achair, but to minimize the impact on airflow, the back‐restandseatofthechairwereremoved.Thissetupiscomparabletothatusedinotherstudies,andthusallowsforcomparisonbetweenthem.Becausetheremainingstructureofthechairhadminimal contactwith themanikin, the influenceofheatconduction could be also ignored. Through various controlmodes,either thesurface temperature,or theheat flux,ora"comfortmode”couldbesimulated.Thelatterisoftenusedintheliteratureandisbasedonthesimpleestimation

Rq skincore (1)

with a default core temperature of 36.4°C and a thermalresistanceofR=0.054m²K/W.Insteadofusingthis“comfortmode”,thesurfacetemperatureofeverysinglebodysegmentwasdeterminedbytheUCBerkeleyThermalComfortModel(Huizenga et al., 2001). Similarly to themodel described byTanabeet al. (2002), this thermoregulationmodel segmentsthehumanbody into16parts. Inaddition,everysegment isdivided into four concentric layers (core, muscle, fat, skin)plus and additional node for heat and moisture transferthroughclothing(Voelkeretal.,2009).Theresultsaremorerealisticthantheestimationbasedonequation(1)andcanbeeasily transmitted from the model to the thermal manikinbecauseofitssimilarsegmentation.

Using theUCBerkeleyThermalComfortModel, thesurfacetemperatureθskinwasdeterminedseparatelyforeachofthechosenboundaryconditions(Figure1&Table1inthesupporting information). Based on this simulation, thesurfacetemperatureofthethermalmanikinwasadjusted.Tosimulate human thermoregulation, an activity level of 1metabolic equivalent (1 met) was selected. The simulationwascarriedoutuntilasteadystatewasreached,usuallyaftert = 2 h. As themeasurements were conductedwith a nudemanikin,clothingwasneglectedinthesimulation(0clo).Asclothing influences the manikin’s surface in terms ofparameters like temperature, emissivity, surface area orsurface texture, the measurements are not directlycomparabletoclothedhumans.

2.2. ParticleStreakTracking

Inorder todetect the speedof theair flow, the two‐dimensional method called Particle Streak Tracking (PST)was used (Dahms et al., 2007). Thewhole system has beendeveloped by the Technische Universität Berlin and can bebought from the spin‐off Human Comfort MeasurementsGmbH. In contrast to anemometry, the advantage of thismethod is the ability to measure and visualize the flowvelocity in a plane. In addition, the measuring equipmentcausesonlyminimaldisturbancetotheflow.Tohelpvisualize

Fig.1.SkintemperatureθskinandheatlossQofthethermalmanikindependingontheoperativeroomtemperatureθop(calculatedbyaveragingairandmeanradianttemperature)

theflow,aspecialgeneratorinjectsabout200bubbles

per second into the climate chamber, each bubble about3mmacross.Tocompensateforthemassofthebubbleskin,the particles are filledwith a heliummixture, whichmakesthemessentiallyequalinweighttotheairandproducesgoodtrackingbehavior.Thekinetic energyof theparticle‐feedingair flow is kept low by four speciallymade diffusers,whichwere arranged on both sides of themeasurement field. Theflowfieldisuniformlyilluminatedinaplanebylight‐emittingdiodes(LED,seealsoFigure2).TheseLEDswerearrangedinordertomakethemeasuringfieldasbrightaspossibleandtominimize the depth of the plane (dmax = 5 cm). In order toreduce light scattering and reflection, the background areawascoveredblack(visibleatwallandceilinginFigure3).

During themeasurements, the lightwaspulsed (longpulse 2∆t, short break ∆t, short pulse ∆t), resulting incharacteristic patterns made by the tracer particle, whichwererecordedwithahighresolutioncamera.Becauseof itsfullframesensor,itshighresolution(21.1megapixels)anditsframe rate (3.9 fps), this camera is capable of documentingair‐flowbytrackingthemovementoftherelativelysmallandsometimesfast‐movingparticles.

Equipped with a lens appropriate for extremely lowlight intensities with a focal length of 24 mm, the cameracapturedanarea in the climate chamberof about0.7x1m(distance to themeasurementplaned=78cm). Inorder toobtainmeaningful, high‐contrast images, the shutter timeofthe camera had to be adjusted to the pulse time. Afterpreliminaryinvestigations,anapertureof2.5atasensitivityof ISO 100, with pulse durations of ∆t = 15; 18; 20 ms forsingle(–‐)ordoubledoublepulses(–‐–‐)wasselected.

The obtained data were then filtered in order tosharpen the contours of the particles and to suppress themeasurementnoise.Inafollowingstep,thegeneratedimagesweresuperimposedontooneanother,sothatamaskcouldbecreatedinwhichnoparticletraceswerecontained.Fromthis


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mask,theindividualimagescouldbesubtracted,leavingonlythetracesoftheparticlesvisiblewithoutabackground.Afterentering the tolerances (length, thickness, radius, size, etc.),the particle velocity was finally calculated based on thedistancebetweenthecentersofthetracesandthepulsetime.

Toincreasethedensityofthemeasuredvalues,upto180 imagespermeasurementweresuperimposedupononeanother.Subsequently,thegeneratedvectorswerecompared

with the original recordings. If necessary, for example, adirection was obviously incorrectly allocated, the vectorswere manually removed. The number of rejected vectorscorrelatedwith the length of the pulse duration. At shorterpulsedurations,notmorethan10%ofthevectorshadtoberemoved. This percentage increased with longer pulsedurations.

Fig.2.Particlestreaktracking:anLEDilluminatesparticlesgeneratedbyabubblegenerator;acontrollerpulsestheLED,whileahigh‐resolutioncamerarecords.

Fig.3.Climatechamberwiththermalmanikin.VisiblearealsothediffusersoftheparticlestreaktrackingandtheflowfieldilluminatedbyLEDs.Inthebackground,severaltemperature

andairvelocitysensorsareinstalled.

To present the flow field vectors in an equidistant grid, theresults were transformed with the help of the followinginterpolationalgorithm:

n

i i

n

iia

ia

xx

vxxxv

12

1,2

1

1

)( (Müller,2000). (2)

A lattice spacing of d = 1 cm was chosen. ThesuperimposingandinterpolationaveragesthePSTmeasuredvaluesintermsofspaceandtime.Withthisstandardizationitis possible to compare the PST measurements with theresults of anemometry. Additionally, the data becomecomparablewithCFDsimulationswhichareusuallybasedonthe Reynolds Averaged Navier Stokes (RANS) equationswhichstatisticallyaveragethefluctuationsintoasteadyflow.

When measuring the air flow using PST, differentmeasurement uncertainties need to be taken into account.Amongothers, this includesmeasurement errors due to theperspective,reducedtracelengthswhentheparticlesenterorleave the light while recording, curved particle pathsespecially resulting from turbulence, particle acceleration,andlackingtrackingbehavioroftheparticles(Müller,2000).Due to theseuncertainties,areliableerrorestimationof thePSTmethodisnearlyimpossible.


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2.3. Thermography

To visualize the temperature profile of the humanbody’s micro climate, infrared thermography was used.Because thermography detects only surface temperatures,but not air temperature, an auxiliary work plane made ofcardboardwasbuiltaroundthe thermalmanikin (Figure4).This allowed us to visualize the corona‐shaped boundarylayerofthethermalmanikinandallowedthetemperaturetobe measured throughout the whole plane, not only at theindividualpointswherethethermistorswerelocated.

Fig.4.Workplanemadeofcardboardsurroundingthethermalmanikin

In preliminary experiments, two different materialsweretested.Thefirstwasacommerciallyavailablemosquitonetmadeofpolyester.Thegoalwastousetheporosityofthenet to keep interferences to flow conditions as low aspossible.However,itwasfoundthatthesurfacetemperatureof the wall behind had an enormous influence on thethermography measurement due to the mesh size of themosquito net. This side effect could not be eliminated, evenwithseveral layersofnetting. Inordertoexcludethiseffect,further studieswere performedwith corrugated cardboard.According to Cehlin et al. (2002), the influence of suchimpervious planes on flow and temperature conditions isnegligible, assuming that they are aligned parallel to thedirectionofairflowatlowflowvelocities.Themeasurementsshowedthatbothrequirementsweremet.

When choosing a material for the auxiliary workplane, it is important to pay attention to its emissioncoefficient. On the camera‐facing side of the cardboard, a

compromise is necessary: on the one hand, an emission ashighaspossibleisdesirabletoreducetheeffectsofreflectivesurrounding surfaces during the recording. On the otherhand, a low emission reduces the influence of the radiativeheat exchange with the surrounding surfaces in the periodleading up to the recording. This allows the auxiliary workplane to achieve the temperature of the air undisturbed byother influences. The cardboard used is a smooth, whitesurfacewithanemissivityofε=0.9.

Inordertolimittheinfluenceofradiationonthesideof the cardboard facing away from the camera, a reflectivecoating (e.g. aluminum foil) canbeapplied to thecardboardto increase the emission coefficient. However, during theseries of measurements we carried out, this was notnecessary, as the 3 mm thick double wave cardboardprovidedagood thermal insulationbetween frontandback.Furthermore,effectsofheatconductionperpendiculartotheboard could be regarded as rather low, since the boundaryconditionsweresimilaronbothsides.

Duringtheexperiment, itwasnecessarytowaituntilthe systemhad reached the thermal steady state,where thesurface temperatureof theauxiliaryworkplane is the sameas the air temperature. This state could be reached quicklybecausethethincardboardhasaverylowheatcapacity.Yetanother advantage of the cardboard used was its physicalstability,allowingittobeinstalledsothatitdidn’tcreateanyobstaclestoflow.Atotaloffourplaneswereconstructed(seealsoFigure4):(1)fromthehipstotheceilingintheplaneofthemeasuring sensors, (2) a horizontalplane extendingoutfromthemid‐lineofthethigh,(3)averticalplaneextendingoutfromthemid‐lineofthelowerleg,and(4)averticalplaneperpendicular to the first plane (not illustrated). As thehorizontal cardboard at the thigh (2) might disturb theupward flow, this plane was only installed when makingseparate measurements. When the cardboard planes weremounted, care was taken to avoid direct contact with thesurface of the thermal manikin in order to reduce heatconductioneffectsorthogonaltothemanikin.

The camera (a Jenoptik Varioscan 3021ST with aStirling cooler) has a resolution of ±0.03K, so thattemperaturegradientscanberepresentedquiteaccuratelyina single thermogram. The previously discussed effects ofradiation from the surrounding surfaces on thethermographic recordingof theauxiliaryworkplane cannotbeexplicitlyquantifiedbecausetheyarehighlydependentontheclimaticboundaryconditionsaswellasthewaytheyareinterpretedbytheuser.Forthisreason,andsimilarlytoPST,this method is appropriate for the visualization ofquantitative data. To control the thermographymeasurements,a temperaturesensorwasmountedontotheboardsurface.

2.4. Anemometryandthermistors

Todouble‐checkthemeasurementsoftheairvelocitywith the PST, additional hot‐wire anemometers with anaccuracyof±1.5%ofthemeasuredvaluewereused(Fig.5).


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During the recordings, statistical fluctuations of the airvelocity couldbemeasured. The thermalmanikin is not thereason for these instabilities, as it is monitored with afrequencyof40Hz.

Fig.5.Temperatureandairvelocitysensorssurroundingthethermalmanikin.

Neither is it the faultof the regulationof the climatechamber, which is more likely to be responsible for longerfluctuations.Therefore,airflowturbulenceandinstabilityarethe only possible causes of the fluctuations. Hence, themeasurementswith the anemometerswere recordedover atimeperiod t=1hwitha sampling intervalof t=1sand thenaveraged. This makes the anemometry measurementscomparabletotheresultsofthestatisticallyaveragedPSTaswellastoaCFDsimulationbasedonRANS.

Tomeasuretheairtemperature,atotalof20NegativeTemperatureCoefficientThermistors(NTC)withanaccuracyof±0.1Kwereused. Inpreliminarymeasurements it turnedout that the heat emitted by the hot‐wire anemometersinfluenced the air temperature. Therefore, the airtemperature and the flow velocity were subsequentlymeasuredseparately.

3. Resultsanddiscussion

3.1. Airvelocity

Figure 6 shows the air velocity, as measured usingPST. In this figure, the original particle tracks are alreadyconverted intovectors indicating thedirectionand speedofthe air‐flow. The high density of vectors is the result of thesuperimpositionofmany individual images(“stacking”).Theeven higher density on the left shoulder is probably due tovery high particle emission from the nearby diffuser. Theupward flow around the shape of the human body isnoticeablyvisible.Clearly,adeadzonecharacterizedonlybysmall‐scale turbulences, is visible directly above the head.Very close to the human body, shear stress slows the

boundarylayer’svelocity.Atthesurfaceoftheskin,theflowvelocity reaches zero, and increases in velocity withincreasingdistancefromthehumanbody.

Fig.6.Particletracksoftheupwardflowofthethermalplume.Thetracksarealreadyconvertedintovectorsindicatingthedirectionandspeedoftheairflow(roomtemperature:18°C)

As the convection is only buoyancy‐driven, the velocitycontinues to increase, eventually reaching a maximum, andthen falling back to the velocity of the undisturbed airsurroundingthebody.Theflowseparationoccursbehindthethickest part of the head (supercritical flow separation),which isa resultof thehighReynoldsandGrashofnumbersduetotheturbulentflowinthisregion.

Totransformtherawdataintoastandardformat,themeasured values were treated with an interpolationalgorithm (eq. 2). In Figure 7, the interpolated results aregiven for the room temperatures of 18°C, 24°C and 30°C.Thanks to the uniform 1 cm interpolation mesh, the airvelocities under the different conditions are easilycomparable. The flow velocity within the micro‐climate isstrongly dependent on the particular boundary conditionsthat result from thedifferences in temperaturebetween thebody and the surrounding air. The higher the temperaturedifferencebetweensurfaceandair temperatures, thehighertheairvelocity.Anairtemperatureof18°C(leftfigure)yieldsamaximumspeedofslightlymorethan0.3m/s,whileat24°Citdecreasesto0.27m/s.At30°C,theairvelocityismeasuredat only 0.2m/s. These findings confirm the numericalsimulationsandmeasurementsdonebyMurakami(2004).

Selected points within the micro‐climate arepresentedinFigure8.Thesearetheresultofthereadings

J.Acoust.Soc.Am.130(6),December2011 Salandinetal.:Noiseinanintensivecareunit

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Fig.7.InterpolatedPSTmeasurements(left:18°C,middle:24°C,right:30°C)showtheinfluenceoftheroomtemperatureonthe

thermalplume

fromtheanemometer,whosepositionswereshowninFigure9.TherawdatafromthePSTmeasurementsarepresentedasapointcloud(PST).Whatisstrikingisthewidedispersionofthedata,whichresultsfromtheturbulenceandinstabilityofthe flow. Therefore, the interpolated values – averaged intermsoftimeandspace–areshownasacurve(PSTavg).Theair velocitymeasured by the anemometers is shown by thelargerdots.Thestandarddeviation(plottedusingerrorbars)showsthemeasuredfluctuations,whichareparticularlyhighdirectlyabovethemanikin.

InFigures8a–8c,theairvelocitymeasuredalongahorizontal line6cmabove thehead (141cmabove floor) isshown at different room temperatures. As shown above inFigure 7, the air velocity above the humanbody is faster atlower room temperatures (Figure 8a) than at higher roomtemperatures (Figure 8c), which is a similar result to thefindingsofLicinaetal.(2013).Thereasonis thatthehigherdensitygradientoftheairproducesmorenaturalconvection.Astheerrorbarsindicate,thestandarddeviationisquitehighdue to the instability and turbulence of the flow. Generally,the upward flow is approximately a shoulder's width wide.The influence of the dead zone directly above the head isrecognizableduetolowerairvelocities.

Figures 8d – 8f illustrate the air velocity measuredalong a vertical line directly above the head. In contrast toFigures8a–8c,thestandarddeviationofthemeasuredflowvelocity ismuch lower. In the area directly above the head,there are almost no particles, due to the dead zone. Withincreasingdistance fromthehead, theairvelocity increases.Again, theairvelocitydependson theroomtemperature,asshowninthedifferencesbetweenFigures8dand8f.Overall,

there is good agreement between the PST and theanemometrymeasurements.

3.2. Airtemperature

Figure 10 shows the results of the thermography atthe uniform wall temperatures of θw=18; 24°C and 30°C.Each picture represents a temperature‐scale of 8K startingfrom thewall temperature.Auniformscale forall the threethermographies could not be used because if it were, nodetailswouldbevisible.

Due to the heat loss of the manikin, the airtemperature is slightly above the wall temperature. Due totheconvectiveheatreleaseofthehuman,thetemperatureofthemicro‐climate is always within a range between that oftheundisturbedairintheroomandthatofthesurfaceofthemanikin. This temperature gradient is responsible for adensity gradient, which is the only reason for the masstransferincaseofnaturalconvection.

While the thermal micro‐climate is very thin in theareaofthelowertorso,itexpandsasonegoesupwards.Thechanging geometry of the body at the neck leads to aparticularly pronounced expansion. In this area, the highestairtemperaturesarealsotobefound.Inaddition,theheadisabarriertothermalbuoyancy.Atthehead,arelativelythickthermal layer can be found, which results in an ascendingplume.Thisplume,whichresultsfromtheascendingflowdueto theheat releaseof thehumanbody,has significant force.The thicknesses of the boundary layers in terms of airtemperatureandintermsofflowvelocityaresimilarbutnotidentical.ThereasonfortheunequalthicknessisthePrandtlnumberPrair~0.7,whichisbasicallytheratiooftheflowto


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Fig.8.Airvelocityhorizontally(y=141cm)abovethehead(a:18°C;b:24°C;c:30°C)andverticallyabovethehead(d:18°C;e:

24°C;f:30°C).TheoriginalPSTmeasurementsarepresentedasapointcloud.Theinterpolatedvalues‐averagedintermsoftimeandspace‐areshownasacurve(PSTavg).Theairvelocitymeasuredbytheanemometersisplottedincludingthestandarddeviation

usingerrorbars

thethermalboundarylayer.Figure11showsthetemperatureprofileoftheplume

asmeasuredbythermistorsalongahorizontallineabovethehead (y=141cm, see also Figure 9) at different roomtemperatures.AswiththeIRmeasurementsinthepreviouslyshownpictures, theair temperaturedirectlyabove theheadisthehighestanddecreaseswithincreasingdistancefromthecenterofthemanikin.Thecoolertheroomtemperature, themore pronounced is the plume. This is the result of the

increasedtemperaturegradientbetweenthemanikinandtheair.

InordertocompareIRandthermistormeasurements,both were measured at several room temperatures and atdifferent spatialpositions,as shown inFigure12. InFigures12a – 12b, the air temperature above the head is shown atdifferentheights.Giventhelevelofaccuracypossibleforeachmeasurementmethod,theresultsofthethermistorsandthethermographymatchwell.


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Fig.9.Technicaldrawingofthetemperature(NTCthermistors)andairvelocitysensors(Anemometers)

There are slight deviations directly over the head,especially at low temperatures. The reason for this is theexposed nature of the location, which is associated withradiationexchangewiththesurroundingarea:atcolderroomtemperatures, the temperature difference betweenwall andauxiliary layer is larger, which means that radiation lossescooldownthesurfaceoftheauxiliarylayer.Furthermore,theradiative heat emitted by the walls is reflected by theauxiliaryplanewhenthethermogramistaken.Theradiationplays a more important role at higher temperaturedifferences. In addition, the standard deviation of themeasuredvaluescorrelateswiththetemperaturedifferences.The higher the temperature difference, the greater thevariationinairtemperatureduetoturbulenceandinstabilityofairflow.

In contrast, Figures 12c – 12d show the verticaltemperature profile of the plume. Here, the first measuringpoint is the surface temperature of the manikin. The airtemperature of the plume falls slowly but visibly with

increasingheight. InFigure12e, thehorizontal temperatureprofile of themicro‐climate at the shoulders (y=107 cm) isshown. The air temperature decreases with increasingdistance from the body until it reaches the value of theundisturbed air temperature. The same applies for thetemperature profile at the hips (y=50cm) in Figure 12f.However, this point seems rather inappropriate forthermography because the convection is influenced by thehorizontalplane(seealsoFigure4).Inaddition,thispointonthe plane exchanges radiationwith other parts of the body(torso,thigh,andarm).Thishasaninfluenceonboththerealaswellasonthemeasuredtemperatureduetothereflectionand absorption of radiation. Therefore, there are slightdiscrepancies in the measured air temperature betweenthermographyandthermistors.

3.3. Heattransfercoefficients

As the thermal manikin is not able to simulateevaporativeheatloss,ourmeasurementsonlyrecordsensibleheat loss. Therefore, the total heat release of the thermalmanikin(Q)consistsofradiativeplusconvectiveheatlosses,whichcanbecalculatedas

askincrskinr hhQ . (3)

The heat release Q, the skin temperature θskin, theradiative temperature θr and the air temperature θa aredeterminedbyourmeasurements,whiletheconvectiveheattransfercoefficienthcandtheradiativecounterparthrarenotmeasureddirectly.Presumably,eachbodyparthasadifferentheattransfercoefficientduetoitsuniqueposition.Ontheonehand,thepositionhasaninfluenceontheradiationexchangewith the environment. On the other hand, convective heatrelease is influenced by the different characteristics of themicro‐climate–itsthickness,airtemperature,airvelocityandturbulence.

The following rearranged equation allows us tocalculatetheconvectiveheattransfercoefficient:

askin

rskinrc

hQh

(4)

The radiative heat transfer coefficient needed in

equation(4)canbedeterminedby(ASHRAE,2005):3

22.2734

rskin

D

rr A

Ah . (5)


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Fig.10.Thermographyoftheworkplane(left:θ=18°C,center:θ=24°C,right:θ=30°C)showstheinfluenceoftheroomtemperatureonthethermalplume

Fig.11.Airtemperatureatdifferentroomtemperatures,asmeasuredbyNTCthermistors.Thecoolertheroom

temperature,themorepronouncedistheplumeduetotheincreasedtemperaturegradientbetweenthemanikinandthe

air

Thissimplifiedequationusestheemissivity(ε=0.95),the Stefan‐Boltzmann constant (σ=5.67∙108W/(m²K4)), theeffectiveradiationareaofthebodyArandtheDuBoissurfaceareaof theskinAD.As theseequationsaregeneral, theycanbeappliedforthewholebodyaswellasforsinglebodyparts.In this study, we used the effective radiation area factorsAr/ADprovidedbyOguroetal.(2002)forallbodysegments.Hence,theradiativeheattransfercoefficienthrwascalculatedfor every single body segment based on its boundaryconditions.

Figure13showsthecalculatedradiativeheattransfercoefficientbasedonequation(5).Duetodifferencesinterms

of skin temperatures as well as in terms of the effectiveradiationarea factor, the radiantheat transferof everybodysegment is different. As expected, the coefficient increasesslightlywithincreasingradiantandskintemperatures.

The convective heat transfer coefficients show theoppositetendency,astheyfallwithincreasingtemperatures.Additionally,thereisalargevariationbetweenthesegments:while fullyexposedsegmentssuchas the footorhandshowquite high coefficients, relatively unexposed segments, suchas the pelvis, have lower ones. At 30°C, for example, thelowestandthehighestcoefficientsvarybyafactorof3.

4. Conclusions

The human body is surrounded by a micro‐climatewhich results from heat release. It consists of a boundarylayer and an ascending plume which both differ greatly intemperature and air velocity from the indoor climatesurroundingthebody.However,thepropertiesofthismicro‐climatearehighlydependentonboundaryconditionsandtheparticularbodysegmentinquestion.Inthisstudy,aseriesofmeasurements was carried out in a climate chamberequippedwithathermalmanikin.Thesurfacetemperatureofthe latter was based on simulations using the UCBerkeleyThermal Comfort Model. For this reason, this approach ismore complex, but might be also more precise than usingfixed temperature values for temperature difference asprovided by de Dear et al. (1997) and Sorensen and Voigt(2003). The micro‐climate of the human body wasinvestigated by combining conventional measurementmethods (anemometry, thermistors) and more innovativetechniques such as thermography and Particle StreakTracking.


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Fig.12.Airtemperaturehorizontallyabovethehead(a:y=141cm;b:y=161cm);verticallyabovethehead(c)andshoulder(d);horizontallyattheshoulder(e)andhips(f).

Although making various simplifications was unavoidable(e.g. breathing), the measurement results are generallycomparable to those with real humans from Craven andSettles(2006).Finally,theconclusionscanbesummarizedasfollows:

Due to the convective heat release, themicro‐climatefeaturestemperaturesinbetweenroomairtemperatureandsurfacetemperatureofthehuman body. This temperature gradient isresponsible for an upward flow of air aroundthehumanbody.The thickestpartandhottesttemperatures are to be found near the head,

which is due to the changing geometry at theshoulders.

Theupwardflowofairresultsinanascendingplume above the head, approximately ashoulder's width wide, but narrowing withincreasingheight.

Skin temperature decreases with falling airtemperature. However, because the skintemperature decreases more slowly, thetemperaturedifferencebetweenskinandroomairtemperatureincreases.

J.Acoust.Soc.Am.130(6),December2011 Salandinetal.:Noiseinanintensivecareunit

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Fig.13.Radiantheattransfercoefficients(left)increaseslightlywithincreasingradiantandskintemperatures.Convectiveheattransfercoefficients(right)showalargevariationbetweenthesegmentsandfallwithincreasingtemperatures.

The lower theair temperature, thegreater the

temperaturedifferencebetweenhumansurfaceand air. The higher the temperature gradient,thehighertheairvelocity.Additionally,thereisaweakcorrelationbetweentheairvelocityandtheturbulenceoftheflow.

Comparing all body segments, the micro‐climate can strongly vary in thickness andshape, air velocity, air temperature andturbulence.

This results in large differences between thebodysegmentsintermsoftheirconvectiveheattransfer coefficient. Additionally, convectiveheat transfer coefficients strongly decreasewithincreasingroomtemperature.

Bycontrast,radiativeheattransfercoefficientsincreasewithincreasingroomtemperatureandshow only minor differences between bodysegments.

The use of thermography and PST provedsuitableforthemeasurementandvisualizationofthemicro‐climate.

When applying PST, general knowledge of theair flow is important, for example in order toselect an appropriate algorithm for temporaland spatial averaging of the measurement

results. Monitoring with conventional sensors(e.g.anemometer)isrecommended.

Thermography is influenced by the radiationexchangewith thesurroundingsurfaceswhichcan influence the measurement results underunfavorable conditions. The control withconventional sensors (e.g. thermistors)provednecessary.

The instability and turbulence of the flownecessitates a time averaging of themeasurements results in order to make theresults comparable with RANS‐based CFDsimulations.

The findings of this paper generate a betterunderstanding of the human body’s micro‐climate. Thedependency of its characteristics from the boundaryconditionsandtheparticularbodysegmentshowthevalueofundertakingsuchadetailed investigation.Thisknowledge isuseful in fields dealingwith thermal comfort, HVAC, indoorair quality and the spread of airborne diseases. Themeasurements can also be used for the validation of CFDsimulations.

5. Acknowledgement

This paper forms part of our work in the researchproject “Methoden und Baustoffe zur nutzerorientierten


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Bausanierung”, which was funded by the German FederalMinistry for Education and Research (BMBF) andadministratedbyProjektträgerJülich.Additionallywehighlyappreciate the cooperation with the Center for the BuiltEnvironment,UCBerkeley.

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measuring the human body’s micro climate using a thermal

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